fix(3696): pr-template enforcer recognises CI/tooling carve-out (#3697)

* fix(3696): pr-template enforcer recognises CI/tooling carve-out

The enforcer in scripts/pr-template-policy.cjs only validated against
three typed templates and three hard-coded DEFAULT_TEMPLATE_MARKERS.
It had zero awareness of the documented CI/tooling/dep/doc-only exception
in .github/pull_request_template.md, meaning external contributors who
followed the documented escape hatch still had their PRs auto-closed.

Fix:
1. Path-scope auto-skip: if every changed file matches a tooling glob
   allowlist (.github/**, scripts/**, docs/**, *.md, .changeset/**,
   dependency manifests), skip enforcement and exit success with no
   comment posted.
2. Explicit exemption marker: if the PR body contains
   <!-- pr-template-exempt: <non-empty reason> -->, skip enforcement.
3. Workflow updated to fetch changed file paths via gh and pass them
   as CHANGED_FILES env to the policy script.
4. Pull request template updated to document both mechanisms and retire
   the old "delete this file" prose carve-out.
5. 14 new tests (TDD red→green); all 25 tests pass.

Closes #3696

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(3696): allow hyphenated pr-template exemption reasons

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/pull_request_template.md

@@ -35,6 +35,7 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md) for the full process.
 
 ---
 
-<!-- If you believe your PR genuinely does not fit any of the above categories (e.g., CI/tooling changes,
-     dependency updates, or doc-only fixes with no linked issue), delete this file and describe your PR below.
-     Add a note explaining why none of the typed templates apply. -->
+<!-- CI/tooling, dependency, and doc-only PRs are auto-detected from the changed file paths — no template
+     needed. For other cross-cutting exempt PRs, paste the line below into your PR body with a non-empty reason:
+     <!-- pr-template-exempt: <reason> -->
+